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The long pause in my postings is due to an unusually heavy workload at Compositex... (what is this "recession" people keep talking about?) I'm not ready for retirement, and the incredible shrinking 401K does not help matters. But I can't complain.. the only thing worse than working is NOT working. Anyway, I will hopefully be less busy with the "day job" so I can devote more time to the Mosi project by the end of May.
I've been going back & forth on my landing gear choices, and this decision impacts the planned Y-tail and engine modifications. I've been stuck on this indecision loop for too long already, and am itching to move ahead. Here's some background on it:
The original Moni monowheel gear (one fixed main wheel near CG, two fixed wingtip wheels, one steerable tailwheel) is OK for the small 33" prop, but I need more ground clearance for the 40" prop, and a taller monowheel leg would drive the need for taller tailwheel and tipwheel legs too... it gets a bit ugly. The monowheel Moni ground handling and tipping forward/prop striking tendency has never received rave reviews from pilots to begin with, and putting it on stilts is bound to make it even worse. My plan for the last few months had been to convert it to a taildragger configuration (fixed dual main wheels ahead of CG with steerable tailwheel), which I still think is the coolest looking and simplest option... but there are drawbacks. Taildragger cons as I see them:
- Higher likelihood of prop strike
- Greater difficulty on ground handling
- Greater tendency to "ground loop"
- Less visibility of runway on takeoff roll
- Longer takeoff roll
- Greater difficulty with takeoffs & landings in cross winds
- Higher likelihood of damage to tail surfaces from runway rocks
- "Ballooning" back up after initial landing touchdown
So, I'm now leaning towards conventional trigear (dual mains behind CG with nosewheel). The original Moni had trigear as an option, but I haven't found much info. on this yet. Final decisions are yet to be made on brakes, steering and gear leg structure. Solid aluminum spring gear legs mounted on the fuselage is one possible answer, but this is a bit heavy and draggy and requires "gun drilling" to get the brake lines through. A streamlined hollow-molded composite spring gear of equivalent strength is certainly a viable option, and is probably what I'll end up doing. Another option is spring loaded gear legs mounted off the main wing spar and not attached to the fuselage at all. This would create less drag due to a smaller exposed frontal area and being located completely out of the propwash, but this option requires tearing into the wing skin. If I was building the wings from scratch, I'd probably do it that way, but I'm not.. so I won't. Even better would be retractable gear... but that's opening an even messier can of worms.
I'm thinking a steerable nosewheel with symmetrical main wheel braking is a much better option than castoring nosewheel and differential braking of main wheels, mostly because of how the brakes need to be adjusted and controlled. Direct coupled nosewheel steering needs to be designed at some rudder-to-wheel steering ratio TBD.. I need to learn more about that to do it right. In any case, changing to trigear on the Mosi increases the labor hours even more, but I think the result will be better overall. -- Change one thing, change EVERYTHING!!
As always, your comments are welcomed.
Many people are irritated by my claim that the climb rate and cruise performance of a KFM107-powered Moni (supposedly 25 hp) can be matched with a 4A032-powered Mosi (supposedly 16 hp). Until the Mosi is flying and generating test data, opinions and beliefs are likely to remain unchanged... hell, even I won't believe it 'til I see it .. but for now I can at least estimate, calculate and predict, so here's a plot of my performance predictions for these two aircraft. The data for this plot is from my spreadsheet-based performance model, reported in pounds of thrust or drag as a function of TAS (true airspeed) reported in miles per hour assuming a 5,000 ft. elevation, which is about what most of the airfields are around here (Salt Lake City area).
Total drag force (see solid lines on the plot) is the sum of induced drag and parasitic drag, and each vary greatly with airspeed. Basically, induced drag is dominant at low speed, while parasitic drag is dominant at high speed. Note that at the lower end of the speed range, the Mosi's greater wing span of 33 feet results in significantly lower induced drag than the Moni's 27.5 ft. wing span. This drag advantage diminishes with increasing airspeed, and above 105 mph, where parasitic drag is greater than induced drag, the Moni's overall drag is lower due to its slightly smaller wing area (75 vs. 84 sq.ft.).
The Moni's thrust was calculated for a standard Moni propeller (33" diameter, 18" pitch) spinning at 6,000 RPM, while Mosi's thrust was figured at 3,600 RPM with a 40" diam., 30" pitch prop. Note that the Moni's thrust is indeed higher over the entire speed range (see large dashes on plot)... but before you jump to conclusions, note that maximum climb rate is determined by thrust minus drag (T-D) at the best climb airspeed. After taking-off and clearing the runway, you attain your maximum climb rate at about the same airspeed used for engine-off minimum sink gliding flight... which is also the same airspeed for maximum endurance or minimum required power to maintain level flight.. they're all the same. Because the Mosi has slightly larger wing area and wing span, it has a best climb airspeed of 48 mph, while the Moni's best climb airspeed is 55 mph. Comparing the T-D curves (see small dashes on plot) of the Moni vs. the Mosi ... note that the Mosi T-D peaks at 48 pounds, while the Moni T-D reaches a maximum of 45 pounds. The result is that the maximum climb rate of the Mosi should be at least as high as the maximum climb rate of the Moni in its standard configuration... That's an encouraging result!
Not shown on the plot is the power delivered by both engines at this best-climb condition.. it's 17.2 bhp for the 4A032 and 19.8 bhp for the KFM107... you see? Getting more with less... more or less...and if you assume the 4A032 can only crank out 16hp max.. using the same 40x30 prop, the Mosi has a maximum T-D of 45 pounds, exactly the same as the KFM107-Moni.
The performance model also predicts maximum airspeed (where T-D equals zero). The KFM107-Moni is shown to reach a top speed of 99 mph with an engine power output of 13.1 bhp, while the 4A032-Mosi is expected to top out at 94 mph, needing 10.4 bhp from the engine... but wait... let's review the assumption of constant propeller RPM. At increasing airspeeds, propellers naturally unload the engine and loose thrust because the angle of attack on the spinning blade airfoils is decreasing. If you keep increasing airspeed without increasing the RPM, the prop will eventually provide zero and then negative thrust as it starts to "pinwheel" while your aircraft goes into a steepening dive. If you want to get to higher airspeed in level flight, it becomes necessary to increase the pitch of the propeller and/or increase the propeller RPMs. The problem is that the Moni's propeller blade tip speed is already at the sonic limit of Mach 0.80 due to the 33" diam. direct-drive prop on the KFM107 engine spinning at a nominal 6,000 RPM. It's generally considered good practice to have propeller blade tip speed at Mach 0.75 or less. Beyond Mach 0.8 tip speed, higher RPMs will result in little or no increase in thrust, but it will result in rapidly increasing fuel consumption and prop noise decibel levels, so there's no good reason for it.
In contrast, the Mosi's 40" diameter prop at 3,600 RPM has a blade tip speed of only Mach 0.59 at 100 mph forward airspeed.. consequently, it should be very quiet. You would have to spin a 40" prop at 5,000 RPM to reach the Mach 0.8 limit. If the 4A032 engine with a 40x30 fixed-pitch prop is spun up from 3,600 to 4,000 RPM, as it should be able to do with a low load on the prop, the Mosi can attain a maximum level speed of 105 mph, which is 6 mph faster than the Moni... This will require 14.1 bhp from the engine and will maintain the blade tip speed at a benign Mach 0.65 .. and with a higher pitched prop, CRUISE EFFICIENCY CAN GET EVEN BETTER!!
But you usually aren't flying around with the prop at maximum RPM. After getting to a desired altitude, you probably want to level off and get somewhere as efficiently as possible. It would be typical to cruise at 60-80 mph to increase your range and fuel economy. To do that on the Mosi requires throttling the engine down to perhaps 2,400-3,200 RPM. Fortunately, 4-stroke engines generally have a wide RPM range for smooth operation, but 2-stroke engines have been known to run rough when they are throttled down below their relatively narrow power band (ironically, 2-stroke engine guys call it "4-stroking" when their engines run rough at lower RPMs). But ignoring that, and just comparing engine power requirements in level cruise at 60-80 mph, the 4A032-powered Mosi requires 5.2 to 8.0 bhp, while the KFM107-powered Moni needs 7.1 to 8.9 bhp.
Therefore, it is expected that with its more efficient wings and propeller, and the flexibility of being able to vary RPM over a broad range without exceeding engine or tip speed Mach number limitations, the 4A032-powered Mosi should be able to match or exceed the climb rate and cruise performance of the KFM107-powered Moni in spite of engine maximum horsepower ratings that would suggest otherwise.
But what about fuel consumption? The 4-stroke 4A032 is expected sip fuel at about one-third to one-half the rate of the KFM107 2-stroke at normal cruise power settings. That ought to keep Al Gore & his cronies off my back.. And what about pollution levels? Not even in the same league! 2-stroke engine exhaust is notoriously dirty, as your sense of smell can easily confirm. With all the increasing emphasis on environmental issues nowadays, it just makes sense to use a quieter, cleaner and more fuel efficient power source... why irritate your friends & neighbors?... it's best to keep them on your side!
I've studied this alternative engine option pretty thoroughly and am satisfied with the engine & propeller plans I've made for the Mosi, at least, so far. But watch out.. if it can go wrong, it will go wrong!.. Murphy and his gremlins are always ready to spoil your best laid plans without warning. If it turns out that in static tests the 4A032 doesn't crank out nearly as much power as I thought ... I'll have to decide whether it's worth it to keep tweaking the engine to squeeze more power out of it, or shift to a different 4-stroke engine option... in any case, I ain't goin' back to no stinkin' 2-stroke!
As always, your comments are welcome.
Here are pictures of a couple of the many dozens of test panels I've made of fiberglass/foam sandwich construction. The S-glass/epoxy composite face sheets are about 0.005" thick, with a 0.25" thick sheet of HighLoad 60 Styrofoam in between. I've been trying to get the best combination of surface preparation for the foam and exterior painted surface finish for the Y-tail and wing tip airfoils, and some of the results have been promising. The picture of the panel on the right shows a mirror-like glossy finish obtained only with run-of-mill aerosol-can white spray lacquer from the hardware store. This was done by applying release agent to a Mylar caul sheet, followed by spray painting it and letting it dry for a few days... then I hand-laminated the sandwich panel with a wet-layup technique, applying the painted side of the caul sheet against the epoxy. After vacuum-bag curing, I peeled off the release-coated caul sheet, leaving a very glossy exterior finish with the paint layer adhering very well to the S-glass/epoxy composite skin. A similar panel that was painted after vacuum-bag curing has much rougher and uglier finish, as seen in the comparison photo on the left (pardon my feeble photography skills). About the only way I could figure out how to capture the finish of these panels was to snap a picture of the reflection from the 4-light bulb fixture in my office.
Composite molders do a similar thing by "gel-coating" the mold before laying up, but by using a glossy caul sheet on the mold side, I can have a not-so-smooth mold surface, and still get a glossy exterior finish... and not just for looks either... you get improved laminar air flow. All in all, it seems clear to me that painting the outside from the inside is the way to go!!! This allows an unskilled painter like me to hide roughness and surface imperfections underneath the paint layer, not on top.
These featherweight panels are amazingly strong and stiff, as the flexural tests are proving now !!... quantitative results will be published here as soon as we have declared victory on achieving our "exit criterion" in the panel design's delivered strength & stiffness. As engineers should know, but sometimes forget, a project's progress can come to a grinding halt in the pursuit of the "optimum".. for example: a directive from your boss and/or customer saying something must be "as light as possible" or some such vague nonsense.. so how do you know when you're done making it lighter?... Remember: nothing is perfect, therefore you can ALWAYS make ANYTHING better... be it stronger, lighter, faster, shinier, more efficient, etc. To exit the endless merry-go-round of incrementally improving your design, you need a finish line.. an exit strategy... a success criterion clearly defined as meeting a pre-established requirement... then you MOVE ON! As one pithy Russian general said it best: 'Better' is the enemy of 'good enough'
As a related side note from my personal experience circa 1999: At Beal Aerospace -- which is now nothing more than a case study on how NOT to run a private space launch company -- we used to joke that the technical requirements were far less important than our wealthy owner's ever-shifting "desirements." He wants the rocket to be bigger one day, simpler the next.. then he would get on a make-it-cheap-like-a-hillbilly-would kick, followed by orders to make it prettier, quicker, tested more, tested less, able to handle even bigger payloads, etc. We were getting good at redesigning & rescheduling, but not building & flying.. Well, the joke was on us.. after burning through untold millions of his money, he laid us all off, shut the place down, and our giant rocketship never flew... lesson learned.
The Moni was originally designed to be powered by the 2-cylinder horizontally-opposed KFM 107 2-stroke engine.. claimed to put out 25 hp at 6,000 RPM with a direct-drive 33" diameter propeller with 18" pitch .. see picture above at left. The small, high-revving prop is inefficient, but actually kind of a clever setup for a motorglider where you're (supposedly) powering up primarily to catch thermal or ridge lift ... you get light weight and a small, low-drag prop for better engine-off soaring performance... but since the Moni design turned out to need a heavy nose ballast weight for proper CG location anyway, the lightweight aspect of the motor was a bit of a joke. And, to be honest, touring motorgliders are flying around like powered airplanes most of the time anyway.. why not enjoy flying even when there's no hope of finding much lift? But above all, personally, I HATE 2-STROKE ENGINES!! They are smelly, polluting, gas-guzzling, noisy, and unreliable ... but they're simple, lightweight, and available in a wide variety of models. I removed the KFM engine from Moni #053 and sold it, so now I'm committed to installing a small 4-stroke engine with direct-drive prop into what will become the Mosi. The engine choices are few and far between, but I'm hoping to emulate what Jim Hardy started doing a few years ago.. see picture on right above.. more details on his website:
New Page
For good reasons, airplane engines like to be horizontally-opposed (AKA boxer) cylinder arrangements, usually 4 or 6 cylinders. So just go out and buy a 70 pound 25hp flat-4 4-stroke engine, right?... I WISH!!.. ain't no such animal to be found. There are a few flat-2s in that power range, like the 1/2 VW, or 1/3 Corvair engines... but they require extensive machining, and they are a bit too heavy, and they tend to shake far more than flat-4s do... the shaking is even worse with in-line 2s. Flat-4s are very well balanced, which is one reason why they are so common in airplanes. The U.S. government surplus market has a few interesting choices.. the 4A032 engine is the one I'm focusing on for now... it's the world's smallest flat-4, only 8 cubic inches displacement per cylinder, 32 cid total. The main problem: it may be a bit TOO small. It was rated at 6hp by Uncle Sam, but that is with a small inlet restriction plate, high altitude, hot, low octane operating conditions. The 4A032 was intended for powering generators, air compressors, welders, etc... designed to run continuously at 3,600 RPM.. and they reportedly last for tens of thousands of hours without major overhaul. A few rather ingenious folks out there are using them to power lawn tractors, log splitters, and who knows what else. Some people claim that it's easy to get 16 hp out of them by just drilling out the restriction plate, and putting on better air filter and exhaust. 16 hp might be enough power for the Mosi, but I'm aiming to get at least 18 hp peak power out of it at 3,600-4,000 RPM while hopefully preserving its high degree of reliability. Whether that's a realistic goal or not, the relevant fact is that the 4A032-equipped Mosi will require much less power than the KFM107-equipped Moni due to the Mosi's larger span and more efficient propeller. A 40x30 prop on a 4A032 engine should be able to match the thrust output of the KFM107 with 33x18 prop across the whole speed range of interest.. that is, if the 4A032 can rev up to 3,400 RPM or so in a static test with that prop.. which ought to require about 17 hp out of that cute little engine... maybe it can do it, maybe not. See the yahoo groups on this and similar government surplus engines:
http://groups.yahoo.com/group/4A032/
http://groups.yahoo.com/group/surplusengines/
There's no shortage of vehement shouts of "that won't work" from "experts," including those that have never even seen a 4A032 engine. In spite of the fact that there is almost no relevant test data on this engine, I'm fairly confident it can be made to work in this application. If it can't, my next choice would be an industrial V-twin like you see on lawn mowers & such. Briggs & Stratton makes a 23hp 627cc (38cid) V-twin that some have used to power airplanes.. see Colomban's MC30 Luciole blog (in French) and the Davis DA-11 which flew with the smaller 18hp B&S V-twin :
La Luciole MC30 ULM Colomban
http://www.aircraft-spruce.com/da11.html
Other more powerful (and heavier) V-twin options include Kohler's CH745, a 725cc 28 hp engine that has electronic fuel-injection and ignition, and Generac's 990cc model for 32+ hp. More info at the small 4-stroke yahoo group:
http://groups.yahoo.com/group/Small4-strokeEngines/
The V-twins generally operate in the right power and RPM band for a direct drive prop, but there are other issues and no easy answers. Potential problems include the vibration, uneven firing sequence (sounds like a Harley motorcycle), and low crankshaft position, which is a challenge for thrust line, cowling and/or prop ground clearance.
So, without the perfect engine waiting on the shelf for me to buy it, the struggle to get the right engine for the Mosi continues... for right now, I plan to finish modifications on the 4A032-4 with a 40" prop. There are some other mods that may improve power & reliability, including turbocharger, new carb/fuel injection, removing the governor, porting and/or shaving the heads, and grinding a new camshaft, but I don't want to get too carried away with trying a bunch of new things until I get some idea on static testing whether or not this engine will put out anything close to enough power to work. Even if it doesn't go on the Mosi, the 4A032 may find it's way onto lighter and "spannier" aircraft, like my favorite ultralight, the Kasperwing... wish I hadn't sold mine... but I digress.
As always, your comments & suggestions are welcome... Happy landings!!
Here 's a peek at how the hingleless or flex hinge looks with the e297 tail airfoils at +/- 21 degree deflection... it's cleaner looking, and simple, but will it work? I was imagining that the width of the flex-hinge would be about 0.25 inches, and the thickness about 0.010 inches.. Material: S-glass cloth/epoxy composite.. perhaps vacuum-bag molded along with the parts. The stiffness of the flex-hinge can be adjusted either by adding extra thin glass cloth plies to increase stiffness, or perhaps cutting a few slots in it with a diamond-wheel Dremel tool to soften it, then taping over it to seal it.
I tried this flex hinge once on my Kestrel mini-UAV project for the Air Force.. but it never flew because I didn't get the stiffness matched to the wimpy servos I was using, and we were running out of money on the contract, plus the little nylon RC hinges were working fine at the time. More on the Kestrel at:
http://home.comcast.net/~compositex/
RC guys use a similar trick called a "living hinge" with a layer of Kevlar fabric molded into the epoxy laminate. After curing, you softened the epoxy around the Kevlar by scoring it with an intentionally dull exacto knife that can't cut through the Kevlar, but the epoxy cracks up.. so it's a soft fabric hinge molded into the hardened airfoil structure, and it works great. There is also a cool tape hinge sort of like a Jacob's ladder with Z-shaped tape strips applied.. probably not suitable for full scale aircraft, but it's a slick idea.
Next post: the engine
I used the CNC foam cutter to carve the e297 airfoil skin molds out of 4” thick blue Styrofoam. The two female mold pieces are for the upper and lower surfaces, while the male piece can be used to mold root ribs and tip ribs. I decided not to use any intermediate ribs and instead rely on the molded shape of the glass-epoxy sandwich panel between the forward and aft spars. The sandwich core material I'm planning to use is Dow’s special Styrofoam grade called High Load 60, which I sliced into flat sheets about 0.25” thick on the CNC foam cutter. This foam is also blue in color, but very different than the run-of-the-mill Dow blue or Owens-Corning pink foam insulation grades you’ll find at home improvement stores. High Load 60 is an extruded foam grade that has its cells stretched in the thickness direction for much higher directional strength and stiffness. It’s rated at 60 psi compressive strength and about 6,000 psi compressive modulus in the all-important thickness direction, as this resists skin buckling far better than common foam grades. The density of High Load 60 is about 2.3 pounds/cu. ft., only slightly higher than the common grades offered at 1.7-2.0 pcf. You might be able to buy High Load 60 from an industrial insulation contractor, since it is most commonly used to insulate the bottoms of heavy refrigerated storage units that impose high compressive loads from their shear weight. If not, you'd have to buy a 100 sheet minimum from the distributor.. not an option for us not-so-rich folks. It's probably easier to just buy Divinycell H45 pre-cut to thickness...I have some of that, and may yet use it... that's good stuff too, almost the same stiffness, and maybe a bit tougher and easier to bond, but you can't hot-wire it!. Stiffened wing skin panels are a sandwich of two plies of 0.005” thick 2x2 twill S-glass cloth impregnated with high-toughness epoxy resin and separated by the 0.25” thick foam core. This sandwich panel wing skin construction is used throughout except around the leading edges, which are solid S-glass/epoxy laminate without any foam core. The whole sandwich panel has about half of the areal density (pounds per square inch) of 0.016” thick aluminum sheet.
I'm not that pleased with how the upper surface hinged ruddervator looks in cross-section.... rather severe bends at the 21 degree max deflection shown in the picture above... I'm afraid the flow will not stay attached, even with turbulator strips and curly Mylar hinge seals . Maybe I'll try a centerline hinge, or a flex-hinge like Zenith Aircraft uses on the ailerons of their metal CH601 Zodiac plane .. sort of a hinge and spring all in one:
http://www.zenithair.com/images/kit-data/ht-aileron.html
I also might use balancing horns on the tips.. thanks to Ted on the brand new "modiflyer" Yahoo group for inspiring me on that with a couple of excellent posts... it seems stability is improved and pilot workload reduced in rough air conditions especially... and I was thinking horns were only for reducing stick forces and flutter.. ( and for Chuck Mangioni ;)
More Y-tail design sketches & composite fabrication reports coming soon.
A few weeks ago, I started laying out the V-tail airfoils on AutoCAD. The non-moving parts are not vertical stabilizers, and they aren't horizontal either.. they are diagonal stabilizers, right? So the diagonal stab. area was increased somewhat from the stock Moni config. to add the extra yaw and pitch stability needed for the span extensions. The hinged area (ruddervators) didn't change much, because it seems that pitch control authority is reportedly ample (even a bit "touchy"), but extra yaw control authority will be added via the 1-2 sq.ft. mini-rudder (the bottom part of the Y-tail). The extra semi-span distance was added to get the pitot tube out of the wider prop wash, since I plan to use a 40" diameter direct-drive prop with a lower-revving (~3,600 RPM) 4-stroke engine. The overall dimension changes on the diagonal tail surfaces are:
-semi-span increased from 35.5" to 40"
-root chord increased from 23.5" to 25.5"
-tip chord decreased from 15" to 13.6"
-hinged ruddervator area approximately unchanged
-forward spar sweep decreased from 13 to 8 degrees
-less swept leading edge, and forward swept trailing edge
After drilling many rivets and removing 7.6 pounds of crumpled aluminum V-tail, I sawed off the extra length of 1.25 x 0.125 aluminum bar for the aft spars so that about 3.5 inches of it was extending outward from the fuselage, same as the forward spar. I bent the forward spar straps forward to accommodate the less-swept leading edge, while keeping the aft spar perpendicular to aircraft centerline... my target weight goal for the Y-tail is under 4 pounds... a savings of at least 3.6 pounds.. that may not sound like much, but it's significant in that the weight of the whole aircraft would drop by at least 14.4 pounds.. It has other advantages too.
Next post: Composite V-tail fabrication and more design stuff.
Moni S/N #053 was brought to flight worthy status in 2006 by EAA Chapter #36 volunteers in Maryland. Looks pretty, huh? It had a standard V-tail built to the original plans, but it was severely damaged in transit to Utah. It ain’t so pretty anymore.
But maybe this shipping damage was a blessing in disguise after all, because at the other end of the aircraft was something that looked rather absurd. Open the cowling and you’ll see a beautiful, shiny, super-lightweight KFM 107 2-stroke engine -- but looking completely out-of-place is a 36 pound bundle of steel bars strapped on top -- nothing but a heavy chunk of inert mass weighing about as much as the engine! At the time, this heavy ballast weight on the nose was considered the best “fix” to locate the aircraft CG correctly, but I figured that making the tail lighter is far more desirable than making the nose heavier. Because the tail has about three times the lever arm distance from the CG as the nose, for every pound removed from the tail, three pounds can and must be removed from the nose ballast, resulting in 4 pounds total weight savings for the whole aircraft. Being an aerospace engineer with 30 years of expertise in composite structures, I was hopelessly attracted to the idea of making the tail lighter with a 4:1 weight savings advantage, and became committed to replacing all that mangled metal with lighter composite tail feathers of my own design.
But weight & balance is not the whole story here. I read up on some flight reports that the Moni has nasty habit of suddenly “flying backwards and upside down” when executing a low speed slip maneuver... Whoa, Nellie! It sounds to me like the V-tail airfoils are stalling, and that’s not a good thing! So, I inspected and measured the V-tail surfaces, and it turns out that the V-tail “airfoil” is not much more than a 5-7% thick flat plate with a rounded leading edge and sharp upper surface hinge line. This configuration seems highly susceptible to leading edge stalls at relatively small angles of attack (like in a slip).
1/11/08 Update: Moni pilots who have experienced this slip-tuck phenomenon are annoyed that I called it a tail stall, and say it is not a sudden occurrence, but if your feet are slow to bring the rudder pedals back to neutral, the nose-down rotation gets worse even with full back stick.. essentially, you run out of "elevator".. so it may not be accurate to say it is a leading edge stall, but there is probably some type of flow separation along with loss of downward lift force generated by the tail at these maxed-out deflections. In any case, more effective tail surfaces would not be a bad thing.
So, it seems reasonable to switch to a thicker, less stall-prone airfoil section for the tail surfaces. I selected Dr. Richard Eppler’s e297, an 11.4% thick symmetrical airfoil section with a plain flap hinged at the 62% chord station. The new V-tail surfaces would have a root chord of 26 inches, a tip chord of 14 inches, and a half span of 40 inches, which is slightly greater in area than the standard Moni design. The plain flap hinges on centerline with a simple radius external surface. The new V-tail airfoils should be lighter, lower in drag, more stall-resistant, and more responsive.
But that’s STILL not the whole story. I read some other flight reports that although the Moni is very responsive to elevator pitch commands, it is a bit sluggish to respond to rudder commands. I figured that a small movable composite rudder surface would enhance the responsiveness to rudder input, and it could easily be installed to replace the fixed aluminum tailcone fairing mounted behind the aft bulkhead. All the swivels and control linkages for this mini-rudder are already there to steer the tailwheel anyway.
So that’s my whale of a tail about how I ended up going down the path of designing and building my own composite Y-tail for what will become the Mosi motorglider… following in the footsteps of the Lear Fan, Waiex, and Xenos aircraft.
Y-tail design details in the next post... Happy Perihelion Day!!
This blog was created to share my ideas on the evolving Mosi Motorglider design and development project. The project is beginning with a series of modifications to John Monnett's innovative Moni motorglider design from the early 1980's. The ingenious all-metal Moni design was produced and sold as a kit back then. New kits have long since become unavailable when INAV went out of business, but the need for a self-launching, high-performance touring motorglider for homebuilders lives on. My eventual goal is to create a completely new design from scratch, but it makes financial and technical sense to me to start with Moni modifications .
Monnett expressed his wish that any extensively modified version of the Moni should not be called a Moni, so the modifications are actually a conversion of Monnett's Moni into Moser's Mosi. An appropriate moniker, wouldn't you say? The Mosi prototype began its life as Moni kit S/N 053, purchased in the early 80's by the late Earl Witt of Pennsylvania. It was finished by a group of volunteers at Maryland EAA Chapter #36, sold to Seabron Smith from Georgia, who sold it to me in 2007. I love the clever design of the Moni as it is, but I was also compelled to redesign it with my own ideas and improvements -- engineers are cursed that way! Here's a list of the major modifications that are now underway:
1. Replace the aluminum V-tail with a composite Y-tail (lighter, less stall-prone, enhanced rudder response).
2. Replace the KFM 107 2-stroke with a 4-stroke engine, perhaps the military surplus 4A032 (world's smallest flat-4) or an industrial V-twin.
3. Increase wing span from 27.5 ft to 33 ft (10 meters) with composite raked wingtips.
4. Convert from monowheel main gear to a taildragger.
I'll be posting progress updates as often as possible. Happy landings to all in 2009!
